13.5 SURFACE TEXTURE DESIGNATION, PRODUCTION, AND QUALITY CONTROL by Ali M.. The surface-texture requirement may be shown at A; the machin-ing allowance at B; the process may be indicate
Trang 1a grinding wheel Removal rates are up to 1.5 in3/h (25 cm3/h) with
prac-tical tolerances on the order of 0.001 in (0.025 mm) A graphite or brass
electrode wheel is operated around 100 to 600 surface ft/min
(30 to 180 m/min) to minimize splashing of the dielectric fluid Typical
applications of this process are in grinding of carbide tools and dies, thin
slots in hard materials, and production grinding of intricate forms
The electrochemical machining (ECM) process (Fig 13.4.22) uses
electrolytes which dissolve the reaction products formed on the
work-piece by electrochemical action; it is similar to a reverse electroplating
process The electrolyte is pumped at high velocities through the tool
A gap of 0.005 to 0.020 in (0.13 to 0.5 mm) is maintained A dc power
supply maintains very high current densities between the tool and the
workpiece In most applications, a current density of 1,000 to 5,000 A
is required per in2of active cutting area The rate of metal removal is
proportional to the amount of current passing between the tool and the
workpiece Removal rates up to 1 in3/min (16 cm3/min) can be obtained
with a 10,000-A power supply The penetration rate is proportional to
the current density for a given workpiece material
The process leaves a burr-free surface It is also a cold machining
process and does no thermal damage to the surface of the workpiece
Electrodes are normally made of brass or copper; stainless steel, titanium,
sintered copper-tungsten, aluminum, and graphite have also been used
The electrolyte is usually a sodium chloride solution up to 2.5 lb/gal
(300 g/L); other solutions and proprietary mixtures are also available
The amount of overcut, defined as the difference between hole
diame-ter and tool diamediame-ter, depends upon cutting conditions For production
applications, the average overcut is around 0.015 in (0.4 mm) The rate
of penetration is up to 0.750 in/min (20 mm/min)
Very good surface finishes may be obtained with this process
However, sharp square corners or sharp corners and flat bottoms cannot
be machined to high accuracies The process is applied mainly to round
or odd-shaped holes with straight parallel sides It is also applied to
cases where conventional methods produce burrs which are costly to
remove The process is particularly economical for materials with a
hardness above 400 HB
The electrochemical grinding (ECG)process (Fig 13.4.23) is a
combi-nation of electrochemical machining and abrasive cutting where most
of the metal removal results from the electrolytic action The process
consists of a rotating cathode, a neutral electrolyte, and abrasive
parti-cles in contact with the workpiece The equipment is similar to a
conventional grinding machine except for the electrical accessories The cathode usually consists of a metal-bonded diamond or aluminum oxide wheel An important function of the abrasive grains is to maintain
a space for the electrolyte between the wheel and workpiece
Surface finish, precision, and metal-removal rate are influenced by the composition of the electrolyte Aqueous solutions of sodium sili-cate, borax, sodium nitrate, and sodium nitrite are commonly used as electrolytes The process is primarily used for tool and cutter sharpen-ing and for machinsharpen-ing of high-strength materials
A combination of the electric-discharge and electrochemical meth-ods of material removal is known as electrochemical discharge grinding (ECDG).The electrode is a pure graphite rotating wheel which electro-chemically grinds the workpiece The intermittent spark discharges remove oxide films that form as a result of electrolytic action The equipment is similar to that for electrochemical grinding Typical appli-cations include machining of fragile parts and resharpening or form grinding of carbides and tools such as milling cutters
In chemical machining (CM)material is removed by chemical or electro-chemical dissolution of preferentially exposed surfaces of the workpiece Selective attack on different areas is controlled by masking or by partial immersion There are two processes involved: chemical millingand chem-ical blanking.Milling applications produce shallow cavities for overall weight reduction, and are also used to make tapered sheets, plates, or extrusions Masking with paint or tapes is common Masking materials may be elastomers (such as butyl rubber, neoprene, and styrene-butadiene)
or plastics (such as polyvinyl chloride, polystyrene, and polyethylene) Typical blanking applications are decorative panels, printed-circuit etch-ing, and thin stampings Etchants are solutions of sodium hydroxide for aluminum, and solutions of hydrochloric and nitric acids for steel
Ultrasonic machining (USM)is a process in which a tool is given
a high-frequency, low-amplitude oscillation, which, in turn, transmits a high velocity to fine abrasive particles that are present between the tool and the workpiece Minute particles of the workpiece are chipped away
on each stroke Aluminum oxide, boron carbide, or silicone carbide grains are used in a water slurry (usually 50 percent by volume), which also carries away the debris Grain size ranges from 200 to 1,000 (see Sec 6 and Figs 13.4.18 and 13.4.19)
The equipment consists of an electronic oscillator, a transducer, a connecting cone or toolholder, and the tool The oscillatory motion is obtained most conveniently by magnetostriction, at approximately 20,000 Hz and a stroke of 0.002 to 0.005 in (0.05 to 0.13 mm) The tool material is normally cold-rolled steel or stainless steel and is brazed, soldered, or fastened mechanically to the transducer through a tool-holder The tool is ordinarily 0.003 to 0.004 in (0.075 to 0.1 mm) smaller than the cavity it produces Tolerances of 0.0005 in (0.013 mm) or bet-ter can be obtained with fine abrasives For best results, roughing cuts should be followed with one or more finishing operations with finer grits The ultrasonic machining process is used in drilling holes, engrav-ing, cavity sinkengrav-ing, slicengrav-ing, broachengrav-ing, etc It is best suited to materials which are hard and brittle, such as ceramics, carbides, borides, ferrites, glass, precious stones, and hardened steels
In water jet machining (WJM),water is ejected from a nozzle at pres-sures as high as 200,000 lb/in2(1,400 MPa) and acts as a saw The process is suitable for cutting and deburring of a variety of materials such as polymers, paper, and brick in thicknesses ranging from 0.03 to
1 in (0.8 to 25 mm) or more The cut can be started at any location, wet-ting is minimal, and no deformation of the rest of the piece takes place Abrasives can be added to the water stream to increase material removal rate, and this is known as abrasive water jet machining (AWJM).
Inabrasive-jet machining(AJM), material is removed by fine abrasive particles (aluminum oxide or silicon carbide) carried in a high-velocity stream of air, nitrogen, or carbon dioxide The gas pressure ranges up
to 120 lb/in2(800 kPa), providing a nozzle velocity of up to 1,000 ft/s (300 m/s) Nozzles are made of tungsten carbide or sapphire Typical applications are in drilling, sawing, slotting, and deburring of hard, brittle materials such as glass
In laser-beam machining (LBM),material is removed by converting electric energy into a narrow beam of light and focusing it on the
ADVANCED MACHINING PROCESSES 13-71
Fig 13.4.22 Schematic diagram of the electrochemical machining process.
Trang 213-72 SURFACE TEXTURE DESIGNATION, PRODUCTION, AND QUALITY CONTROL
workpiece The high energy density of the beam is capable of melting
and vaporizing all materials, and consequently, there is a thin
heat-affected zone The most commonly used laser types are CO2(pulsed
or continuous-wave) and Nd:YAG Typical applications include
cut-ting a variety of metallic and nonmetallic materials, drilling (as small
as 0.0002 in or 0.005 mm in diameter), and marking The efficiency
of cutting increases with decreasing thermal conductivity and
reflec-tivity of the material Because of the inherent flexibility of the process,
programmable and computer-controlled laser cutting is now becom-ing important, particularly in cuttbecom-ing profiles and multiple holes of various shapes and sizes on large sheets Cutting speeds may range up
to 25 ft/ min (7.5 m/min)
The electron-beam machining (EBM) process removes material by focusing high-velocity electrons on the workpiece Unlike lasers, this process is carried out in a vacuum chamber and is used for drilling small holes, scribing, and cutting slots in all materials, including ceramics
13.5 SURFACE TEXTURE DESIGNATION, PRODUCTION, AND QUALITY CONTROL
by Ali M Sadegh
R EFERENCES : American National Standards Institute, “Surface Texture,” ANSI/
ASME B 46.1-1985, and “Surface Texture Symbols,” ANSI Y 14.36-1978.
Broadston, “Control of Surface Quality,” Surface Checking Gage Co., Hollywood,
CA ASME, “Metals Engineering Design Handbook,” McGraw-Hill SME,
“Tool and Manufacturing Engineers Handbook,” McGraw-Hill.
Rapid changes in the complexity and precision requirements of
mechan-ical products since 1945 have created a need for improved methods of
determining, designating, producing, and controlling the surface texture
of manufactured parts Although standards are aimed at standardizing
methods for measuring by using stylus probes and electronic
transduc-ers for surface quality control, other descriptive specifications are
some-times required, i.e., interferometric light bands, peak-to-valley by optical
sectioning, light reflectance by commercial glossmeters, etc Other
para-meters are used by highly industrialized foreign countries to solve their
surface specification problems These include the high-spot counter and
bearing area meter of England (Talysurf ); the total peak-to-valley, or R1,
of Germany (Perthen); and the R or average amplitude of surface
devia-tions of France In the United States, peak counting is used in the
sheet-steel industry, instrumentation is available (Bendix), and a standard for
specification, SAE J-911, exists
Surface texture control should be considered for many reasons,
among them being the following:
1 Advancements in the technology of metal-cutting tools and
machinery have made the production of higher-quality surfaces possible
2 Products are now being designed that depend upon proper quality
control of critical surfaces for their successful operation as well as for
long, troublefree performance in service
3 Remote manufacture and the necessity for controlling costs have
made it preferable that finish requirements for all the critical surfaces of
a part be specified on the drawing
4 The design engineer, who best understands the overall function of
a part and all its surfaces, should be able to determine the requirement
for surface texture control where applicable and to use a satisfactory
standardized method for providing this information on the drawing for
use by manufacturing departments
5 Manufacturing personnel should know what processes are able to
produce surfaces within specifications and should be able to verify that
the production techniques in use are under control
6 Quality control personnel should be able to check conformance to
surface texture specifications if product quality is to be maintained and
product performance and reputation ensured
DESIGN CRITERIA
Surfaces produced by various processes exhibit distinct differences in
tex-ture These differences make it possible for honed, lapped, polished,
turned, milled, or ground surfaces to be easily identified As a result of its
unique character, the surface texture produced by any given process can
be readily compared with other surfaces produced by the same process
through the simple means of comparing the average size of its irregulari-ties, using applicable standards and modern measurement methods It is then possible to predict and control its performance with considerable certainty by limiting the range of the average size of its characteristic sur-face irregularities Sursur-face texture standards make this control possible Variations in the texture of a critical surface of a part influence its ability to resist wear and fatigue; to assist or destroy effective lubrica-tion; to increase or decrease its friction and/or abrasive action on other parts, and to resist corrosion, as well as affect many other properties that may be critical under certain conditions
Clay has shown that the load-carrying capacity of nitrided shafts of varying degrees of roughness, all running at 1,500 r/min in diamond-turned lead-bronze bushings finished to 20 min (0.50 mm), varies as shown in Fig 13.5.1 The effects of roughness values on the friction between a flat slider on a well-lubricated rotating disk are shown in Fig 13.5.2 Surface texture control should be a normal design consideration under the following conditions:
1 For those parts whose roughness must be held within closely con-trolled limits for optimum performance In such cases, even the process may have to be specified Automobile engine cylinder walls, which should be finished to about 13 min (0.32 mm) and have a circumferential (ground) or an angular (honed) lay, are an example If too rough, exces-sive wear occurs, if too smooth, piston rings will not seat properly, lubri-cation is poor, and surfaces will seize or gall
2 Some parts, such as antifriction bearings, cannot be made too smooth for their function In these cases, the designer must optimize the tradeoff between the added costs of production and various benefits derived from added performance, such as higher reliability and market value
Fig 13.5.1 Load-carrying capacity of journal bearings related to the surface
Trang 3DESIGNATION STANDARDS, SYMBOLS, AND CONVENTIONS 13-73
unsound to specify too smooth a surface as to make it too rough—or to control it at all if not necessary Wherever normal shop practice will produce acceptable surfaces, as in drilling, tapping, and threading, or in keyways, slots, and other purely functional surfaces, unnecessary sur-face texture control will add costs which should be avoided
Whereas each specialized field of endeavor has its own traditional criteria for determining which surface finishes are optimum for ade-quate performance, Table 13.5.1 provides some common examples for design review, and Table 13.5.6 provides data on the surface texture ranges that can be obtained from normal production processes
DESIGNATION STANDARDS, SYMBOLS, AND CONVENTIONS
The precise definition and measurement of surface texture irregularities
of machined surfaces are almost impossible because the irregularities are very complex in shape and character and, being so small, do not lend themselves to direct measurement Although both their shape and length may affect their properties, control of their average height and direction usually provides sufficient control of their performance The standards
do not specify the surface texture suitable for any particular application, nor the means by which it may be produced or measured Neither are the standards concerned with other surface qualities such as appearance, lus-ter, color, hardness, microstructure, or corrosion and wear resistance, any of which may be a governing design consideration
The standards provide definitions of the terms used in delineating crit-ical surface-texture qualities and a series of symbols and conventions suitable for their designation and control In the discussion which fol-lows, the reference standards used are “Surface Texture” (ANSI/ ASME B46.1-1985) and “Surface Texture Symbols” (ANSI Y 14.36-1978) The basic ANSI symbol for designating surface texture is the check-mark with horizontal extension shown in Fig 13.5.3 The symbol with the triangle at the base indicates a requirement for a machining
allowance, in preference to the old f symbol Another, with the small
circle in the base, prohibits machining; hence surfaces must be pro-duced without the removal of material by processes such as cast, forged, hot- or cold-finished, die-cast, sintered- or injection-molded, to name a few The surface-texture requirement may be shown at A; the machin-ing allowance at B; the process may be indicated above the line at C;
Table 13.5.1 Typical Surface Texture Design Requirements
Clearance surfaces Rough machine parts Mating surfaces (static) Chased and cut threads Clutch-disk faces Surfaces for soft gaskets Piston-pin bores Brake drums Cylinder block, top Gear locating faces Gear shafts and bores Ratchet and pawl teeth Milled threads Rolling surfaces Gearbox faces Piston crowns Turbine-blade dovetails
Broached holes Bronze journal bearings Gear teeth
Slideways and gibs Press-fit parts Piston-rod bushings Antifriction bearing seats Sealing surfaces for hydraulic tube fittings
Motor shafts Gear teeth (heavy loads) Spline shafts O-ring grooves (static) Antifriction bearing bores and faces Camshaft lobes
Compressor-blade airfoils Journals for elastomer lip seals Engine cylinder bores Piston outside diameters Crankshaft bearings Jet-engine stator blades Valve-tappet cam faces Hydraulic-cylinder bores Lapped antifriction bearings Ball-bearing races Piston pins Hydraulic piston rods Carbon-seal mating surfaces Shop-gage faces
Comparator anvils Bearing balls Gages and mirrors
Fig 13.5.2 Effect of surface texture on friction with hydrodynamic lubrication
using a flat slider on a rotating disk Z
ft/min; P 2
3 There are some parts where surfaces must be made as smooth as
possible for optimum performance regardless of cost, such as gages,
gage blocks, lenses, and carbon pressure seals
4 In some cases, the nature of the most satisfactory finishing process
may dictate the surface texture requirements to attain production
effi-ciency, uniformity, and control even though the individual performance of
the part itself may not be dependent on the quality of the controlled
sur-face Hardened steel bushings, e.g., which must be ground to close
toler-ance for press fit into housings, could have outside surfaces well beyond
the roughness range specified and still perform their function satisfactorily
5 For parts which the shop, with unjustified pride, has traditionally
finished to greater perfection than is necessary, the use of proper
sur-face texture designations will encourage rougher sursur-faces on exterior
and other surfaces that do not need to be finely finished Significant cost
reductions will accrue thereby
It is the designer’s responsibility to decide which surfaces of a given
part are critical to its design function and which are not This decision
should be based upon a full knowledge of the part’s function as well as
of the performance of various surface textures that might be specified
From both a design and an economic standpoint, it may be just as
Trang 413-74 SURFACE TEXTURE DESIGNATION, PRODUCTION, AND QUALITY CONTROL
the roughness width cutoff (sampling length) at D, and the lay at E The
ANSI symbol provides places for the insertion of numbers to specify a
wide variety of texture characteristics, as shown in Table 13.5.2
Control of roughness,the finely spaced surface texture irregularities
resulting from the manufacturing process or the cutting action of tools
or abrasive grains, is the most important function accomplished through
the use of these standards, because roughness, in general, has a greater
effect on performance than any other surface quality The
roughness-height index value is a number which equals the arithmetic average
deviation of the minute surface irregularities from a hypothetical
per-fect surface, expressed in either millionths of an inch (microinches, min,
0.000001 in) or in micrometres, mm, if drawing dimensions are in
met-ric, SI units For control purposes, roughness-height values are taken
from Table 13.5.3, with those in boldface type given preference
The term roughness cutoff, a characteristic of tracer-point measuring
instruments, is used to limit the length of trace within which the
asperi-ties of the surface must lie for consideration as roughness Asperity
spac-ings greater than roughness cutoff are then considered as waviness
Wavinessrefers to the secondary irregularities upon which roughness
is superimposed, which are of significantly longer wavelength and are
usually caused by machine or work deflections, tool or workpiece
vibration, heat treatment, or warping Waviness can be measured by a
dial indicator or a profile recording instrument from which roughness
has been filtered out It is rated as maximum peak-to-valley distance
and is indicated by the preferred values of Table 13.5.4 For fine
wavi-ness control, techniques involving contact-area determination in percent
(90, 75, 50 percent preferred) may be required Waviness control by
interferometric methods is also common, where notes, such as “Flat
within XX helium light bands,” may be used Dimensions may be deter-mined from the precision length table (see Sec 1)
Layrefers to the direction of the predominant visible surface rough-ness pattern It can be controlled by use of the approved symbols given
in Table 13.5.5, which indicate desired lay direction with respect to the boundary line of the surface upon which the symbol is placed
Flawsare imperfections in a surface that occur only at infrequent inter-vals They are usually caused by nonuniformity of the material, or they result from damage to the surface subsequent to processing, such as scratches, dents, pits, and cracks Flaws should not be considered in surface texture measurements, as the standards do not consider or classify them Acceptance or rejection of parts having flaws is strictly a matter of judgment based upon whether the flaw will compromise the intended function of the part
To call attention to the fact that surface texture values are specified on any given drawing, a note and typical symbol may be used as follows:
Surface texture per ANSI B46.1 Values for nondesignated surfaces can be limited by the note
All machined surfaces except as noted
MEASUREMENT
Two general methods exist to measure surface texture: profile methodsand
area methods.Profile methods measure the contour of the surface in a plane usually perpendicular to the surface Area methods measure an area
of a surface and produce results that depend on area-averaged properties
2xx
2
Fig 13.5.3 Application and use of surface texture symbols.
Table 13.5.2 Application of Surface Texture Values to Surface Symbols
Machining is required to produce the sur-face The basic amount of stock provided for machining is specified at the left of the short leg of the symbol Specify in millimetres (inches).
Removal of material by machining is prohibited.
Lay designation is indicated by the lay symbol placed at the right of the long leg Roughness sampling length or cutoff rating
is placed below the horizontal extension When no value is shown, 0.80 mm is as-sumed Specify in millimetres (inches) Where required, maximum roughness spac-ing shall be placed at the right of the lay symbol Any lesser rating shall be accept-able Specify in millimetres (inches).
Roughness average rating is placed at the left of the long leg The specification of only one rating shall indicate the maxi-mum value and any lesser value shall be acceptable Specify in micrometres (microinches).
The specification of maximum value and minimum value roughness average ratings indicates permissible range of value rating.
Specify in micrometres (microinches).
Maximum waviness height rating is placed above the horizontal extension Any lesser rating shall be acceptable Specify in millimetres (inches).
Maximum waviness spacing rating is placed above the horizontal extension and to the right of the waviness height rating Any lesser rating shall be acceptable Specify
Trang 5SURFACE QUALITY VERSUS TOLERANCES 13-75
Another categorization is by contact methodsand noncontact methods.
Contact methods include stylus methods (tracer-point analysis) and
capacitance methods Noncontact methods include light section
microscopy, optical reflection measurements, and interferometry
Replicas of typical standard machined surfaces provide less accurate
but often adequate reference and control of rougher surfaces with R a
over 16 min
The United States and 25 other countries have adopted the roughness averageR aas the standard measure of surface roughness (See ANSI/ ASME B46.1-1985.)
PRODUCTION
Various production processes can produce surfaces within the ranges shown in Table 13.5.6 For production efficiency, it is best that critical areas requiring surface texture control be clearly designated on drawings
so that proper machining and adequate protection from damage during processing will be ensured
SURFACE QUALITY VERSUS TOLERANCES
It should be remembered that surface quality and tolerances are dis-tinctly different attributes that are controlled for completely separate purposes Tolerancesare established to limit the range of the size of a part at the time of manufacture, as measured with gages, micrometres,
Table 13.5.5 Lay Symbols
Table 13.5.3 Preferred Series Roughness Average Values R a, mm and min
Table 13.5.4 Preferred Series Maximum Waviness Height Values
Trang 613-76 SURFACE TEXTURE DESIGNATION, PRODUCTION, AND QUALITY CONTROL
or other traditional measuring devices having anvils that make contact
with the part Surface qualitycontrols, on the other hand, serve to limit
the minute surface irregularities or asperities that are formed by the
manufacturing process These lie under the gage anvils during
mea-surement and do not use up tolerances.
QUALITY CONTROL (SIX SIGMA)
Quality control is a system that outlines the policies and procedures
necessary to improve and control the various processes in
manufacturing that will ultimately lead to improved business
performance
Six Sigmais a quality management program to achieve “six sigma”
levels of quality It was pioneered by Motorola in the mid-1980s and
has spread to many other manufacturing companies In statistics,
sigma refers to the standard deviation of a set of data Therefore, six sigmarefers to six standard deviations Likewise, three sigmarefers to three standard deviations In probability and statistics, the standard deviation is the most commonly used measure of statistical disper-sion; i.e., it measures the degree to which values in a data set are spread The standard deviation is defined as the square root of the variance, i.e., the root mean square (rms) deviation from the average
It is defined in this way to give us a measure of dispersion Assuming that defects occur according to a standard normal distrib-ution, this corresponds to approximately 2 quality failures per million parts manufactured In practical application of the six sigma methodol-ogy, however, the rate is taken to be 3.4 per million
Initially, many believed that such high process reliability was impos-sible, and three sigma (67,000 defects per million opportunities, or DPMO) was considered acceptable However, market leaders have measurably reached six sigma in numerous processes
Table 13.5.6 Surface-Roughness Ranges of Production Processes
Trang 713.6 WOODCUTTING TOOLS AND MACHINES
by Richard W Perkins
R EFERENCES : Davis, Machining and Related Characteristics of United States
Hardwoods, USDA Tech Bull 1267 Harris, “A Handbook of Woodcutting,” Her
Majesty’s Stationery Office, London Koch, “Wood Machining Processes,”
Ronald Press Kollmann, Wood Machining, in Kollmann and Côté, “Principles of
Wood Science and Technology,” chap 9, Springer-Verlag.
SAWING
Sawing machines are classified according to basic machine design, i.e.,
band saw, gang saw, chain saw, circular saw Saws are designated as
ripsawsif they are designed to cut along the grain or crosscutsaws if
they are designed to cut across the grain A combinationsaw is designed
to cut reasonably well along the grain, across the grain, or along a
direc-tion at an angle to the grain (miter).Sawing machines are often further
classified according to the specific operation for which they are used,
e.g., headsaw(the primary log-breakdown saw in a sawmill), resaw(saw
for ripping cants into boards), edger (saw for edging boards in a
sawmill), variety saw(general-purpose saw for use in furniture plants),
scroll saw(general-purpose narrow-band saw for use in furniture plants)
The thickness of the saw blade is designated in terms of the
Birmingham wire gage (BWG) (see Sec 8.2) Large-diameter [40 to 60
in (1.02 to 1.52 mm)] circular-saw blades are tapered so that they are
thicker at the center than at the rim Typical headsaw blades range in
thickness from 5 to 6 BWG [0.203 to 0.220 in (5.16 to 5.59 mm)] for use
in heavy-duty applications to 8 to 9 BWG [0.148 to 0.165 in (3.76 to
4.19 mm)] for lighter operations Small-diameter [6 to 30 in (152 to 762
mm)] circular saws are generally flat-ground and range from 10 to 18 BWG
[0.049 to 0.134 in (1.24 to 3.40 mm)] in thickness Band-saw and
gang-saw blades are flat-ground and are generally thinner than circular-gang-saw
blades designed for similar applications For example, typical
wide-band-saw blades for sawmill use range from 11 to 16 BWG [0.065 to
0.120 in (1.65 to 3.05 mm)] in thickness The thickness of a band-saw
blade is determined by the cutting load and the diameter of the band
wheel Gang-saw blades are generally somewhat thicker than band-saw
blades for similar operations Narrow-band-saw blades for use on scroll
band saws range in thickness from 20 to 25 BWG [0.020 to 0.035 in
(0.51 to 0.89 mm)] and range in width from to about 1 in (3.17 to
44.5 mm) depending upon the curvature of cuts to be made
The considerable amount of heat generated at the cutting edge results
in compressive stresses in the rim of the saw blade of sufficient
magni-tude to cause mechanical instability of the saw blades Circular-saw
blades and wide-band-saw blades are commonly prestressed (or
tensioned) to reduce the possibility of buckling Small circular-saw
blades for use on power-feed rip-saws and crosscut saws are frequently
provided with expansion slotsfor the same purpose
The shape of the cutting portion of the sawtoothis determined by
speci-fying the hook, face bevel, top bevel, and clearance angles The optimum
tooth shape depends primarily upon cutting direction, moisture content,
and density of the workpiece material Sawteeth are, in general, designed
in such a way that the portion of the cutting edge which is required to cut
across the fiber direction is provided with the maximum effective rake
angle consistent with tool strength and wear considerations Ripsaws are
designed with a hook angle between some 46 for inserted-tooth circular
headsaws used to cut green material and 10 for solid-tooth saws cutting
dense material at low moisture content Ripsaws generally have zero face
bevel and top bevel angle; however, spring-set ripsaws sometimes are
provided with a moderate top bevel angle (5 to 15) The hook angle for
crosscut saws ranges from positive 10 to negative 30 These saws are
generally designed with both top and face bevel angles of 5 to 15;
how-ever, in some cases top and face bevel angles as high as 45 are employed
A compromise design is used for combination saws which embodies the
3⁄4
1⁄8
features of both ripsaws and crosscut saws in order to provide a tool which can cut reasonably well in all directions The clearance angle should be maintained at the smallest possible value in order to provide for maximum tooth strength For ripsawing applications, the clearance angle should be about 12 to 15 The minimum satisfactory clearance angle is determined by the nature of the work material, not from kinematical con-siderations of the motion of the tool through the work In some cases of cutoff, combination, and narrow-band-saw designs where the tooth pitch
is relatively small, much larger clearance angles are used in order to pro-vide the necessary gullet volume
A certain amount of clearance between the saw blade and the gener-ated surface (side clearanceor set) is necessary to prevent frictional heat-ing of the saw blade In the case of solid-tooth circular saws and band
or gang saws, the side clearance is generally provided either by deflect-ing alternate teeth (spring-setting) or by spreading the cutting edge
(swage-setting).The amount of side clearance depends upon density, moisture content, and size of the saw blade In most cases, satisfactory
results are obtained if the side clearance S is determined from the for-mula S [in (mm)]
(BWG) of the saw blade, f (n) sponding to the gage number n, and A has values from Table 13.6.1.
Certain specialty circular saws such as planer, smooth-trimmer, and miter saws are hollow-ground to provide side clearance Inserted-tooth saws, carbide-tipped saws, and chain-saw teeth are designed so that suf-ficient side clearance is provided for the life of the tool; consequently, the setting of such saws is unnecessary
Table 13.6.1 Values of A for Computing Side Clearance
Workpiece material Specific gravity Specific gravity less than 0.45 greater than 0.55
Circular rip and combination 0.90 1.00 0.85 0.95
The tooth speedfor sawing operations ranges from 3,000 to 17,000 ft/ min (15 to 86 m/s) approx Large tooth speeds are in general desirable
in order to permit maximum work rates The upper limit of permissible tooth speed depends in most cases on machine design considerations and not on considerations of wear or surface quality as in the case of metal cutting Exceptionally high tooth speeds may result in charring of the work material, which is machined at slow feed rates
In many sawing applications, surface qualityis not of prime impor-tance since the sawed surfaces are subsequently machined, e.g., by planing, shaping, sanding; therefore, it is desirable to operate the saw at the largest feed per tooth consistent with gullet overloading Large val-ues of feed per tooth result in lower amounts of work required per unit volume of material cut and in lower amounts of wear per unit tool travel Large-diameter circular saws, wide-band saws, and gang saws for rip-ping green material are generally designed so that the feed per tooth should be about 0.08 to 0.12 in (2.03 to 3.05 mm) Small-diameter cir-cular saws are designed so that the feed per tooth ranges from 0.03 in (0.76 mm) for dense hardwoods to 0.05 in (1.27 mm) for low-density softwoods Narrow-band saws are generally operated at somewhat smaller values of feed per tooth, e.g., 0.005 to 0.04 in (0.13 to 1.02 mm)
Trang 813-78 WOODCUTTING TOOLS AND MACHINES
Smaller values of feed per tooth are necessary for applications where
surface quality is of prime importance, e.g., glue-joint ripsawing and
variety-saw operations The degree of gullet loading is measured by the
gullet-feed index (GFI),which is computed as the feed per tooth times the
depth of face divided by the gullet area The maximum GFI depends
primarily upon species, moisture content, and cutting direction It is
generally conceded that the maximum GFI for ripsawing lies between
0.3 for high-density, moisture-content material and 0.4 for
low-density, high-moisture-content material For specific information, see
Telford, For Prod Res Soc Proc., 1949.
Saws vary considerably in design of the gullet shape.The primary
design considerations are gullet area and tooth strength; however,
spe-cial design shapes are often required for certain classes of workpiece
material, e.g., for ripping frozen wood
Materials Saw blades and the sawteeth of solid-tooth saws are
gen-erally made of a nickel tool steel The bits for inserted-tooth saws were
historically plain carbon tool steel; however, high-speed steel bits or
bits with a cast-alloy inlay (e.g., Stellite) are sometimes used in
appli-cations where metal or gravel will not be encountered Small-diameter
circular saws of virtually all designs are made with cemented-carbide
tips This type is almost imperative in applications where highly
abra-sive material is cut, namely, in plywood and particleboard operations
Sawing Power References: Endersby, The Performance of Circular
Plate Ripsaws, For Prod Res Bull 27, Her Majesty’s Stationery Office,
London, 1953 Johnston, Experimental Cut-off Saw, For Prod Jour.,
June 1962 Oehrli, Research in Cross-cutting with Power Saw Chain
Teeth, For Prod Jour., Jan 1960 Telford, Energy Requirements for
Insert-point Circular Headsaws, Proc For Prod Res Soc., 1949.
An approximate relation for computing the power P, ft lb/min (W),
required to saw is
where k is the kerf, in (m); v is the tooth speed, ft /min (m/s); p is the
tooth pitch, in (m); A and B are constants for a given sawing operation,
lb/in (N/m) and lb/in2(N/m2), respectively; and t ais the average chip
thickness, in (m) The average chip thickness is computed from the
rela-tion t a t d/b, where f t is the feed per tooth; d is the depth of face;
b is the length of the tool path through the workpiece; and g has the
value 1 except for saws with spring-set or offset teeth, in which case g
has the value 2 The constants A and B depend primarily upon cutting
direction (ripsawing, crosscutting), moisture content below the
fiber-saturation point and specific gravity of the workpiece material, and
tooth shape The values of A and B (see Table 13.6.2) depend to some
degree upon the depth of face, saw diameter, gullet shape, gullet-feed
index, saw speed, and whether the tool motion is linear or rotary; how-ever, the effect of these variables can generally be neglected for pur-poses of approximation
Computers are now utilized in sawmills where raw logs are first processed into rough-cut lumber With suitable software, the mill oper-ator can input key dimensions of the log and receive the cutting pattern which provides a mix of cross sections of lumber so as to maximize the yield from the log The saving in waste is sizable, and this technique is especially attractive in view of the decreasing availability of large-caliper old stand timber, together with the cost of same
PLANING AND MOLDING Machinery Planing and molding machines employ a rotating cutter-head to generate a smooth, defect-free surface by cutting in a direction approximately along the grain A surfacer (or planer)is designed to machine boards or panels to uniform thickness A facer (orfacing planer)
is designed to generate a flat (plane) surface on the wide faces of boards The edge jointeris intended to perform the same task on the edges of boards in preparation for edge-gluing into panels A planer-matcheris a heavy-duty machine designed to plane rough boards to uni-form width and thickness in one operation This machine is commonly used for dressing dimension lumber and producing millwork The
molderis a high-production machine for use in furniture plants to gen-erate parts of uniform cross-sectional shape
Recommended Operating Conditions It is of prime importance to adjust the operating conditions and knife geometry so that the machin-ing defects are reduced to a satisfactory level The most commonly encountered defects are torn (chipped) grain, fuzzy grain, raised and loosened grain, and chip marks Torn grainis caused by the wood split-ting ahead of the cutsplit-ting edge and below the generated surface It is generally associated with large cutting angle, large chip thickness, low moisture content, and low workpiece material density The fuzzy-grain
defect is characterized by small groups of wood fibers which stand up above the generated surface This defect is caused by incomplete sever-ing of the wood by the cuttsever-ing edge and is generally associated with small cutting angles, dull knives, low-density species, high moisture content, and (often) the presence of abnormal wood known as reaction wood The raised-graindefect is characterized by an uneven surface where one portion of the annual ring is raised above the remaining part
Loosened grainis similar to raised grain; however, loosened grain is characterized by a separation of the early wood from the late wood which is readily discernible to the naked eye The raised- and loosened-grain defects are attributed to the crushing of springwood cells as the
Table 13.6.2 Constants for Sawing-Power Estimation
Species gravity content, % Anglese situation lb/in N/m lb/in 2 10 –3 N/m 2 10 –6
aEndersby.
bJohnston.
cHoyle, unpublished report, N.Y State College of Forestry, Syracuse, NY, 1958.
dOehrli.
eThe numbers represent hook angle, face bevel angle, and top bevel in degrees.
fCutting performed on frozen material.
Trang 9knife passes over the surface (Edge-grain material may exhibit a defect
similar to the raised-grain defect if machining is performed at a markedly
different moisture content from that encountered at some later time.)
Raised and loosened grains are associated with dull knives, excessive
jointing of knives [the jointing land should not exceed in (0.79 mm)],
and high moisture content of workpiece material Chip marksare caused
by chips which are forced by the knife into the generated surface as the
knife enters the workpiece material Chip marks are associated with
inadequate exhaust, low moisture content, and species (e.g., birch,
Douglas fir, and maple have a marked propensity toward the
chip-mark defect)
Depth of cutis an important variable with respect to surface quality,
particularly in the case of species which are quite prone to the
torn-grain defect (e.g., hard maple, Douglas fir, southern yellow pine) In
most cases, the depth of cut should be less than in (1.59 mm) The
number of marks per inch (marks per metre)(reciprocal of the feed per
cutter) is an important variable in all cases; however, it is most
impor-tant in those cases for which the torn-grain defect is highly probable
The marks per inch (marks per metre) should be between 8 and 12
(315 and 472) for rough planing operations and from 12 to 16 (472 to
630) for finishing cuts Slightly higher values may be necessary for
refractory (brittle) species or for situations where knots or curly grain
are present It is seldom necessary to exceed a value of 20 marks per
inch (787 marks per metre) The clearance angleshould in all cases
exceed a value of 10 When it is desired to hone or joint the knives
between sharpenings, a value of about 20 should be used The
opti-mum cutting anglelies between 20 and 30 for most planing situations;
however, in the case of interlocked or wavy grain, low moisture content,
or species with a marked tendency toward the torn-grain defect, it may
be necessary to reduce the cutting angle to 10 or 15
BORING
Machinery The typical general-purpose wood-boring machine has
a single vertical spindle and is a hand-feed machine Production
machines are often of the vertical, multiple-spindle, adjustable-gang
type or the horizontal type with two adjustable, independently driven
spindles The former type is commonly employed in furniture plants for
boring holes in the faces of parts, and the latter type is commonly used
for boring dowel holes in the edges and ends of parts
Tool Design A wide variety of tool designs is available for
special-ized boring tasks; however, the most commonly used tools are the
taper-head drill, the spur machine drill, and the machine bit The taperhead
drill is a twist drill with a point angle of 60 to 90, lip clearance angle
of 15 to 20, chisel-edge angle of 125 to 135, and helix angle of 20
to 40 Taper-head drills are used for drilling screw holes and for
bor-ing dowel holes along the grain The spur machinedrill is equivalent to
a twist drill having a point angle of 180 with the addition of a
pyrami-dal point (instead of a web) and spurs at the circumference These drills
are designed with a helix angle of 20 to 40 and a clearance angle of 15
to 20 The machine bithas a specially formed head which determines
the configuration of the spurs It also has a point Machine bits are
designed with a helix angle of 40 to 60, cutting angle of 20 to 40, and
clearance angle of 15 to 20 Machine bits are designed with spurs
con-tiguous to the cutting edges (double-spur machine bit), with spurs
removed from the vicinity of the cutting edges (extension-lip machine bit),
and with the outlining portion of the spurs removed (flat-cut machine bit).
The purpose of the spurs is to aid in severing wood fibers across their
axes, thereby increasing hole-wall smoothness when boring across the
grain Therefore, drills or bits with spurs (double-spur machine drill and
bit) are intended for boring across the grain, whereas drills or bits
with-out spurs (taper-head drill, flat-cut machine bit) are intended for boring
along the grain or at an angle to the grain
Taper-head and spur machine drills can be sharpened until they
become too short for further use; however, machine bits and other bit
styles which have specially formed heads can only be sharpened a
lim-ited number of times before the spur and cutting-face configuration is
significantly altered Since most wood-boring tools are sharpened by
1⁄16
1⁄32
filing the clearance face, it is important to ensure that sufficient clear-ance is maintained The clearclear-ance angle should be at least 5 greater than the angle whose tangent (function) is the feed per revolution divided by the circumference of the drill point
Recommended Operating Conditions The most common defects are tearing of fibers from the end-grain portions of the hole surface and charring of hole surfaces Rough hole surfaces are most often encoun-tered in low-density and ring-porous species This defect can generally
be reduced to a satisfactory level by controlling the chip thickness Charring is commonly a problem in high-density species It can be avoided by maintaining the peripheral speed of the tool below a level which depends upon density and moisture content and by maintaining the chip thickness at a satisfactory level Large chip thickness may result in excessive tool temperature and therefore rapid tool wear; how-ever, large chip thickness is seldom a cause of hole charring The fol-lowing recommendations pertain to the use of spur-type drills or bits for boring material at about 6 percent moisture content across the grain For species having a specific gravity less than 0.45, the chip thickness should be between 0.015 and 0.030 in (0.38 and 0.76 mm), and the peripheral speed of the tool should not exceed 900 ft/min (4.57 m/s) For material of specific gravity between 0.45 and 0.65, satisfactory results can be obtained with values of chip thickness between 0.015 and 0.045 in (0.38 and 1.14 mm) and with peripheral speeds less than 700 ft/ min (3.56 m/s) For material of specific gravity greater than 0.65, the chip thickness should lie between 0.015 and 0.030 in (0.38 and 0.76 mm) and the peripheral speed should not exceed 500 ft/min (2.54 m/s) Somewhat higher values of chip thickness and peripheral speed can be employed when the moisture content of the material is higher
SANDING
(See also Secs 6.7 and 6.8.)
Machinery Machines for production sanding of parts having flat sur-faces are multiple-drum sanders, automatic-stroke sanders, and wide-belt sanders Multiple-drumsanders are of the endless-bed or rollfeed type and have from two to six drums The drum at the infeed end is fitted with a relatively coarse abrasive (40 to 100 grit), takes a relatively heavy cut [0.010 to 0.015 in (0.25 to 0.38 mm)], and operates at a relatively slow surface speed [3,000 to 3,500 ft/min (15.24 to 17.78 m/s)] The drum at the outfeed end has a relatively fine abrasive paper (60 to 150 grit), takes
a relatively light cut [about 0.005 in (0.13 mm)], and operates at a some-what higher surface speed [4,000 to 5,000 ft/min (20.3 to 25.4 m/s)]
Automatic-strokesanders employ a narrow abrasive belt and a reciprocat-ing shoe which forces the abrasive belt against the work material This machine is commonly employed in furniture plants for the final white-sanding operation prior to finish coating The automatic-stroke sander has
a relatively low rate of material removal (about one-tenth to one-third of the rate for the final drum of a multiple-drum sander) and is operated with
a belt speed of 3,000 to 7,500 ft/min (15.2 to 38.1 m/s) Wide-beltsanders are commonly used in board plants (plywood, particle board, hardboard) They have the advantage of higher production rates and somewhat greater accuracy than multiple-drum sanders [e.g., feed rates up to 100 ft/min (0.51 m/s) as opposed to about 35 ft/min (0.18 m/s)] Wide-belt sanders operate at surface speeds of approximately 5,000 ft/min (25.4 m/s) and are capable of operating at depths of cut of 0.006 to 0.020 in (0.15 to 0.51 mm) depending upon workpiece material density
Abrasive Tools The abrasive tool consists of a backingto carry the
abrasiveand an adhesivecoat to fix the abrasive to the backing Backings are constructed of paper, cloth, or vulcanized fiber or consist of a cloth-paper combination The adhesive coating (see also Sec 6) is made up
of two coatings; the first coat (make coat)acts to join the abrasive mate-rial to the backing, and the second coat (size coat)acts to provide the necessary support for the abrasive particles Coating materials are gen-erally animal glues, urea resins, or phenolic resins The choice of mate-rial for the make and size coats depends upon the required flexibility of the tool and the work rate required of the tool Abrasive materials (see also Sec 6) for woodworking applications are garnet, aluminum oxide, and silicon carbide is the most commonly used abrasive mineral
SANDING 13-79
Trang 1013-80 PRECISION CLEANING
because of its low cost and acceptable working qualities for
low-work-rate situations It is generally used for sheet goods, for sanding
soft-woods with all types of machines, and for sanding where the belt is
loaded up (as opposed to worn out) Aluminum oxideabrasive is used
extensively for sanding hardwoods, particleboard, and hardboard
Silicon carbideabrasive is used for sanding and polishing between
coat-ing operations and for machine sandcoat-ing of particleboard and hardboard
Silicon carbide is also frequently used for the sanding of softwoods
where the removal of raised fibers is a problem The size of the abrasive
particles is specified by the mesh number (the approximate number of openings per inch in the screen through which the particles will pass) (See Commercial Std CS217-59, “Grading of Abrasive Grain on Coated Abrasive Products,” U.S Government Printing Office.) Mesh numbers range from about 600 to 12 Size may also be designated by
an older system of symbols which range from 10/0 (mesh no 400) through 0 (mesh no 80) to 4 (mesh no 12) Some general recom-mendations for common white wood sanding operations are presented
in Table 13.6.3
1⁄2
13.7 PRECISION CLEANING
by Charles Osborn IMPORTANCE OF CLEANLINESS
Long ignored, and still somewhat underestimated, the importance of
part cleanliness is rapidly coming to the fore in today’s high-tech
products, especially as we head further into nanotechnologies
Historically many manufacturers have dismissed part cleaning as an
insignificant part of the process Many learn all too late that their highly engineered, closely toleranced device is rendered inoperable
by a tiny particle, often so small that it can’t be seen with the naked eye Suddenly they are faced with a steep learning curve, for myriad equipment, chemistry, staffing, and environmental issues await them This material will help you become familiar with some of the
Table 13.6.3 Recommendations for Common Whitewood Sanding Operations
Abrasive
Wide-belt
Mold sanding
* May be single- or multiple-grit operation.
† First number for cutting-down operations, second number for finishing operations.
N OTE : G
S OURCE: Graham, Furniture Production, July and Aug., 1961; and Martin, Wood Working Digest, Sept 1961.
80–220* 80–220* 80–220* 80–220* 280–400 24–150* 100–150 80; 120† 100; 150–180† 100; 150–180† 80; 120† 80; 120† 100; 150† 100; 150† 60; 100† 60–150* 60–150* 80–120* 80–120* 80–120* 80–120*